safe_mode definitive documentation

I've looked through all the docs and I can only find bits and pieces reagrding safe_mode. I know that part of what safe_mode does is allow php scripts to only execute things in the safe_mode_bin_dir. Can anyone tell me exactly what checks safe_mode goes through in PHP4? Even better if anyone knows where in the PHP source these checks (specifically regarding the writing of files) are done that would be even more helpful. A thousand thanks, -Joe S. joes@th.net